Steroid hormone receptors, including androgen receptor (AR), glucocorticoid receptor (GR), and the progesterone receptor (PR), require the ordered assembly of various chaperone and co-chaperone proteins in order to reach a functional state. The final stage in the receptor maturation process requires the formation of a multimeric complex consisting of an Hsp90 dimer, p23, and one of several large immunophilins. Previous studies have demonstrated that (i) the large immunophilin, FK506-binding protein 52 (FKBP52), acts to potentiate GR, AR, and PR receptor signaling pathways, and (ii) FKBP52-mediated regulation of receptor function appears to be localized to the receptor hormone binding domain. In cellular studies, FKBP52 has been shown to preferentially regulate GR, AR, and PR receptor-mediated signal transduction. See, for example, Cheung-Flynn et al., Mol. Endocrinol., 19:1654-66 (2005); Riggs et al., EMBO J., 22:1158-67 (2003); and Tranguch et al., J. Clin. Invest., 117:1824-34 (2007). Given its receptor specificity, FKBP52 represents an attractive therapeutic target for the treatment of hormone-dependent diseases.
To date, the only known compounds for inhibition of AR function are related to selective AR modulators that bind to the hormone binding pocket, and are therefore competitive inhibitors of endogenous hormone binding. It has been shown that when certain molecules bind to the BF3 region of the AR hormone binding domain they can generally inhibit AR function in the 100 μM range. See, Estebanez-Perpina et al., Proc. Arad Acad. Sci. USA, 104:16074-79 (2007). However, there still exists a need for compounds which are selective AR modulators which are not competitive agonists or antagonists to endogenous hormone binding.
Androgens are a major stimulator of prostate tumor growth and all current therapies act as classic antagonists by competing with androgens for binding the AR hormone binding pocket. This mechanism of action exploits the dependence of AR for hormone activation and current treatment options are essentially ineffective once androgen-dependence is lost.
Thus, the direct targeting of FKBP52 with small molecules will lead to a more potent drug with the potential to simultaneously hit a variety of targets known to have, or suspected of having, a role in prostate cancer.